1. Field of the Invention
The present invention relates to a piezoelectric oscillator, and in particular to a piezoelectric oscillator in which a current of a piezoelectric resonator is reduced.
2. Prior Art
In recent years, mobile radio communication equipment and transmission communication equipment are reduced in size, and are required highly-performance. With these requirements, it is also required miniaturization and high-stabilization to a piezoelectric oscillator utilized in these devices as a frequency control device. A piezoelectric oscillator is comprised of a piezoelectric vibrator, a frequency trimming circuit, and a frequency-temperature controlled circuit.
A piezoelectric vibrator is an electro-mechano-vibrator, and it has a high reliability against aged deterioration since an electric current flows in the piezoelectric vibrator (hereinafter referred to as “vibrator current”) is a little. FIG. 4 shows an example of a conventional Colpitts oscillator using a silicon transistor. A serial circuit of capacitor Cb and capacitor Ce, which serve as a load capacitor for the Colpitts oscillator, is connected between base of a transistor TR11 and ground.
A point between the capacitors Cb and Ce is connected with emitter of the transistor TR11, and the point is grounded via resistor Re. In addition, a base bias circuitry comprising resistor RB 11 and R12 is connected to the base of transistor TR11. A series circuit comprised of piezoelectric vibrator Xtal and capacitor C11 is connected between the base of the transistor TR11 and ground. A collector of the transistor TR11 is connected to a source voltage Vcc line.
Value of resistors, capacitors of the Colpitts oscillator are set as shown in FIG. 4, and frequency of the piezoelectric vibrator Xtal is of 10 MHz, signal levels are 537 mVrms at the point A, 296 mVrms at the point B, and 229 mVrms at the point C, respectively. Power current (current consumption of an oscillating circuit) is 7.14 mA at the oscillation.
There is a Colpitts oscillator having a silicon transistor connected between a collector of the transistor TR11 and the source voltage line Vcc. Capacitor Cb between the base and emitter of the transistor TR11 is in range of 0 pF to 100 pF in this circuit, a vibrator current ix is increased as the value of the capacitor Cb. The vibrator current ix maintains constant when the value of the capacitor Cb is above 100 pF. According to the experiment result, the highest vibrator current ix was 6500 μA.
Another way to suppress the vibrator current, there is a Colpitts oscillator in which a base current of a transistor is controlled by an AGC circuit. A vibrator current is suppressed since the AGC circuit lowers a gain of the transistor. However, the Colpitts oscillator using the AGC circuit has the problem that miniaturization of the oscillator and low cost cannot be achieved.
A conventional Colpitts oscillator without an AGC circuit, a suppression of a vibrator current is restricted since the vibrator current increases with increment of capacity between base and emitter of the transistor. On the other hand, using an AGC circuit can lower the vibrator current. However, circuitry becomes complicated, and miniaturization is difficult.
The object of the present invention is to provide a small size piezoelectric oscillator capable of suppressing vibrator current by simple circuit configuration, and having a favorable aging characteristic.